The present invention relates to a process for determining the stability of a water-hydrocarbon emulsion.
This process can be used to determine the stability of a water-hydrocarbon emulsion which is stable at ambient temperature, generally usable as a fuel, which, under the influence of a variation in temperaturexe2x80x94cooling or heatingxe2x80x94is capable of separating into two or more liquid and/or solid phases on account of demixing or crystallization of the water, followed or preceded by sedimentation of the paraffins in the hydrocarbon matrix.
In the text hereinbelow, the term xe2x80x9cemulsionxe2x80x9d or xe2x80x9cwater-hydrocarbon emulsionxe2x80x9d will denote, without preference, an emulsion of an aqueous dispersed phase in hydrocarbons and the possible additives thereof constituting the continuous phase, or alternatively an emulsion of hydrocarbons dispersed in an aqueous phase.
It is well known that the presence of a small fraction of water dispersed in a hydrocarbon improves the quality of combustion of this hydrocarbon and substantially reduces the amount of harmful, unburnt and nitrogen oxides emissions, the vaporization of the water resulting in a lowering of the temperature in the combustion chamber. Unfortunately, the immiscibility of the two fluids substantially limits the use of this property to its implementation in burners preparing the emulsion in situ. Attempts to produce fuels and combustion spirits consisting of an emulsion by addition of surfactants to the mixture failed, since they were not sufficiently stable for industrial application. Recent investigations have allowed the formulation of novel fuels whose stability is such that their industrial exploitation appears possible (see patent application WO 97/34969 of Mar. 17, 1997).
This industrial application requires the development of a reliable process for controlling the stability of the emulsions thus manufactured, this process being reliable both over time and under the influence of temperature.
The problem is difficult on account of the complex phenomena which take place in a medium, which is heterogeneous by nature, in particular when it is subjected to variations in temperature.
The reason for this is that crude or refined hydrocarbons contain a larger or smaller proportion of paraffins, which are soluble xe2x80x9cunder hot conditionsxe2x80x9d but which, under the influence of a decrease in temperature, can crystallize and then sediment and thus give rise to dysfunctions on storage or during their use. The stability of the emulsion is temperature-sensitive both under hot conditions, since an increase in temperature promotes the demixing phenomenon, and under cold conditions, in which case the crystallization of the free water accelerates the separation process.
Thus, the possibility of providing conditions under which a liquid emulsion, which is initially stable at ambient temperature, can separate into at least two phases under the influence of time and/or temperature is a considerable asset for the optimum use of this emulsion.
The emulsion can be prepared with any hydrocarbon, such as spirits, gas oils, domestic fuel oils or heavy fuel oils, these fuels possibly containing various additives or components known to those skilled in the art, such as oxygenated compounds (alcohols, ethers or methyl esters of plant oil). The same types of problem arise for all the products, particularly products containing paraffins, for which filtration, pumping and blockage problems are observed, in particular in motors and in industrial and domestic heating systems. By analogy, reference will be made to summer or winter emulsion formulations, as common terminology, for domestic fuel oils, summer fuel oil and winter fuel oil according to the specifications in force.
Surfactant additives which facilitate the formation of the emulsion and ensure its stability are added to the water-hydrocarbon mixture to avoid the appearance of the demixing phenomenon. To avoid the crystallization and then sedimentation of the paraffins, during use under cold conditions, additives whose action delays the appearance of the crystals, prevents their development, keeps them in suspension or prevents their sedimentation are added to the emulsions already containing their own additives. It is thus important to measure the impact of these various additives on the phenomena of phase separation of an emulsion.
Various methods exist for measuring the characteristics of appearance and separation of a solid phase in liquid.
A first method is based on measuring the weight of the solids, such as the paraffins in the gas oils which have crystallized at a given temperature. These paraffins are extracted from the hydrocarbon by centrifugation (patent EP-0,355,053 A2) or by aggregation in a gravity sedimenter (U.S. Pat. No. 4,357,244). These tests make it possible only to determine the total amount of paraffins which have crystallized and which can sediment out. They gave a measure of the excess sedimentation.
A second type of test simulates the real-time sedimentation in small tanks (standard NF M 07-085) in which are stored hydrocarbons at low temperature for 24 or 48 hours. The appearance and volume of each phase are then assessed visually by the experimenter, in particular the position of the interface between the two phases. These tests give an approximate qualitative measure of the sedimentation.
Optical methods for measuring the characteristics of appearance of two immiscible-liquid or solid-liquid phases also exist. Mention may be made of patent FR 2,577,319 which is directed towards determining the cloud point of gas oils, and patent FR 2,681,428 which is directed towards the demixing of two liquids (measurement of the aniline point of hydrocarbons).
These methods all have drawbacks and inadequacies:
They are long, since they generally last 24 hours or 48 hours.
They are not reliable, since they depend only on subjectivity of the observer.
Most especially, however, they do not make it possible to measure the amounts of the separated phases, or to know the speed of separation of the phases, or even to explain and quantify the successive states through which the liquid passes when the temperature changes.
The process for determining the stability of a water-hydrocarbon emulsion by thermogravimetric analysis, which is the subject of the invention, solves the problem of the quantitative measurement of the separation of the liquid or solid immiscible phases using a liquid which has been made homogeneous.
A subject of the present invention is a process for determining the stability of a water-hydrocarbon emulsion liable to exhibit a phase separation, characterized in that
in a first step, by subjecting the said emulsion to a suitable heat treatment, it is brought to a predetermined test temperature and the variation in the apparent weight P of the gravimetric detector, a portion of which is immersed in the emulsion, is continuously measured by thermoaravimetry, then
in a second step, the emulsion is maintained at this temperature while continuously measuring the variation in the apparent weight of the said detector by thermogravimetry, and the curve of the variation of this weight is recorded simultaneously, and then
the mass of the separated phase collected, on the one hand, and the speed of separation of the phases corresponding to the slope of the said curve, mainly the speed measured at the breaking point corresponding to a substantial and continuous increase in the apparent weight P at the start of the second step, on the other hand, are determined from the said curve, and
the stability of the emulsion is deduced by comparison with known reference emulsions, whose stability over time has been corroborated by tests of long-lasting stability.
The expression xe2x80x9cpredetermined temperaturexe2x80x9d means here the steady temperature at which it is desired to measure the stability of the emulsion, but also, for the behaviour under cold conditions, the temperature at which the separation is visible, i.e. detectable to the naked eye or by infrared as described in patents FR 2,577,319 and FR 2,681,428.
The process of the invention will be carried out according to two main variants depending on whether it is directed towards the stability at a predetermined temperature above that of the crystallization of water or, exceptionally, of certain heavy paraffins (behaviour under hot conditions), or the stability at a predetermined temperature which is below the crystallization temperature of at least one of the constituents behaviour under cold conditions). The profiles of the curves for the variation in the apparent weight of the detector as a function of time and of temperature show substantial differences in the duration of the various steps whether the stability of the emulsion is monitored under hot or cold conditions.
The reason for this is that the stability under hot conditions leads to a first step whose duration is associated with the difference in temperature between the temperature of the emulsion prepared, i.e. generally close to ambient temperature, and the test steady temperature. If the test is carried out at ambient temperature, this duration can be zero. If the test temperature is greater than the initial temperature of the emulsion, the latter will have to be heated. On the other hand, the second step, which is complete when the variation in weight becomes zero (i.e. when the phases are completely separate), can be very long, especially if a particularly stable emulsion is tested. In this case, the speed of separation will be the predominant factor to be taken into account.
In order to assess the temperature behaviour of the emulsions, the predetermined test temperature is between 10 and 70xc2x0 C., the emulsion being brought to this temperature at a heating or cooling speed, from ambient temperature, generally of between 0.05 and 10xc2x0 C./min.
The determination of the stability of an emulsion at low temperature consists in monitoring the crystallization and sedimentation of water, on the other hand, and of the paraffins, on the other hand, in an emulsion.
In a first embodiment, the first step consists in gradually lowering the temperature at a speed generally of between 0.05 and 10xc2x0 C./min to between the crystallization temperatures of the water and of the paraffins, while continuously recording the variation in the apparent weight of the detector. This weight decreases on account of the increase in the density of the emulsion. During the second step, the change in the apparent weight of the detector is recorded, while keeping the temperature constant. This weight remains substantially constant up to the point of crystallization of one or other of the two phases depending on whether the crystallization temperature of water is less than or greater than that of the paraffins.
In a second embodiment, the first step consists or gradually lowering the temperature at a speed generally of between 0.05 and 10xc2x0 C./min down to a predetermined temperature which is less than the crystallization temperatures of the paraffins and of water but greater than the flow temperature of the hydrocarbon-based mixture.
The advantages of the process, which is the subject of the invention, are the precision, reliability and reproducibility of the results obtained, both for assessing the speed of separation of the phases and for measuring the weight variations of the separated phases.
A subject of the present invention is also a device for measuring the separation of an emulsion into several liquid and/or solid phases, comprising a thermogravimetric balance fitted with a gravimetric detector, the portion of which immersed in a tank (2) containing the said emulsion is a crucible (5), the said tank being connected to a cooling circuit, the said device being characterized in that the crucible is free, preferably coaxial with the tank whose cylindrical cross section is such that the ratio of the largest diameter of the crucible to the diameter of the tank is between 0.1 and 0.9.
The crucible has a cylindrical shape comprising a base and rims, the height of which does not exceed the level of liquid in the tank. The height of the rims is between 0.5 mm and 30 mm and generally equal to 5 mm.
The characteristics of the present device will become more apparent on examination of FIGS. 1A, 1B and 1C and the description thereof below.
The device represented in FIG. 1A comprises a thermogravimetric beam balance (1) (of SETARAM type), a tank (2) containing the macroscopically homogeneous liquid mixture (3) to be studied, a temperature control device (not represented in the diagram) for cooling or heating the tank and a computer system (not represented in the diagram) for recording and processing the data.
The beam (4) of the balance (1) carries, suspended on the left arm in the diagram, a crucible (5) immersed in the tank (2) containing the mixture. The tank (2) has a jacket (6) and allows the temperature of the mixture to be modified, by means of a heating or cooling circuit, not represented in the diagram.
The crucible (5) has a cylindrical shape, like the tank, and comprises a base and rims (7).
A standard optical and magnetic system (10), combined with the balance, allows the variations in the weight of the crucible to be measured and recorded.